Hydrogen has been recognized as an environmentally friendly clean fuel of the future since it has various applications in power generation systems. For example, hydrogen can be used as a fuel for combustion engines, gas turbines, fuel cells, especially proton exchange membrane fuel cells, which use hydrogen and air to produce electricity, generating only water as a by-product. Fuel cells are being developed to replace traditional electricity generators because they produce clean, environmentally friendly energy. However, these fuel cells require external supply and storage devices for hydrogen. Extensive efforts have been made to develop a safe and efficient way to store hydrogen, especially in mobile applications. Conventional hydrogen storage technologies include liquid hydrogen, compressed gas cylinders, dehydrogenation of compounds, chemical adsorption into metal alloys and chemical storage as hydrides. However, each of these systems is either hazardous or bulky.
There are various prior art hydrogen generation systems that utilize chemical hydrides. One type of hydrogen generation system employs chemical hydrides in solid phase, e.g. granules. U.S. Pat. No. 5,372,617, comprises a closed vessel for mixing chemical hydride powder together with water. The water is introduced into the vessel through an inlet. The vessel contains a mechanical stirring device to ensure adequate contact between the powder and the water, and to prevent the powder from clumping. The hydrogen gas is removed through an outlet in the vessel, and is supplied directly to the fuel cell. These systems tend to be inefficient since the stirring mechanism consumes energy, and increases the overall weight and complexity of the system. Furthermore, the noise generated by the stirring is undesirable. In addition, the reaction rate tends to be low, making the hydrogen generation unpredictable and thus hard to control. The systems also tend to be large and cumbersome.
Another similar hydrogen generation system is disclosed in U.S. Pat. No. 5,702,491. The hydrogen generation system substantially comprises a thermally isolated container for containing chemical hydride, a preheater to heat the chemical hydride to a predetermined temperature before the chemical hydride is hydrolysed, a water pipe to supply water into the container to generate hydrogen. This system entails adiabatic arrangement and heating devices, hence results in lower energy efficiency and complicated structure.
U.S. Pat. No. 5,833,934 discloses a cartridge-type reactor comprising a storage compartment for storing chemical hydride particles, a water absorbent material for retaining water and a water distribution tube for introducing water into the mass of chemical hydride particles. Other cartridge arrangements can be found in, for example, U.S. Pat. Nos. 4,261,956, 5,514,353. Although the cartridge generator in U.S. Pat. No. 5,833,934 provides some improvement over prior art generator concepts, it still suffers, as all the above-mentioned generators, from poor thermal management of the reactor, and hence little if any control of reaction rate. The heating effects associated with the chemical hydride reaction, which is exothermic, can in turn positively or negatively affect the reaction rate and efficiency. Temperature plays an important role in chemical hydride reactions. It directly affects the reaction rate. Poor thermal management of the reactor may lead to undesirable reaction rate, deactivation of catalyst, production of unwanted by-product, and in extreme cases, clogging or damage to the reactor.
Another method of generating and storing hydrogen has been recently disclosed in WO 01/51410. This method uses a chemical hydride solution, such as NaBH4, as a hydrogen storage medium. Generally, chemical hydride reacts with water in the presence of a catalyst to generate hydrogen, as shown in the equation below:NaBH4+2H2O→4H2+NaBO2+HEAT
The chemical hydride acts as both the hydrogen carrier and the storage medium. Ruthenium, Cobalt, Platinum or any alloys thereof may be used to catalyze the above reaction. It is noted that hydrogen is liberated from both the borohydride (NaBH4) solution and the water. The borohydride solution is relatively cheap, and is much easier and safer to handle and transport than liquid or pressurized hydrogen. As a result, there are a number of advantages associated with using borohydride as a method of storing hydrogen as a fuel for use in fuel cells.
WO 01/51410 discloses a system, where an aqueous chemical hydride solution contained in a vessel is brought into contact with a catalyst disposed in a containment system to generate hydrogen. However, there are still a number of problems associated with this liquid phased system. In particular, the reaction in the vessel is not regulated. The temperatures of the solution and catalyst are not uniform, resulting in unstable reaction rate and poor ability to respond in real time to the fuel (hydrogen) needs of the hydrogen consuming devices, such as fuel cells or the like. This ability is referred to as load following ability. Moreover, it is also difficult to control the amount of catalyst in contact with the chemical hydride solution, which makes it even more difficult to control the reaction.
Therefore, there remains a need for a chemical hydride reaction system and reactor which offer improved control of the reaction rate by providing improved thermal management of the hydride solution and more uniform contact between catalyst and chemical hydride solution.